... Let B bisect the chord ZZ'. Then, by geometry, the angle FQY is equal to the angle BTF, and the protuberance FY is equal the sine of that angle, making QF radius. This angle, made by the axis of the vortex and the surface of the sphere, is commonly between 30d and 40d, according as the moon is near her apogee ... — Outlines of a Mechanical Theory of Storms - Containing the True Law of Lunar Influence • T. Bassnett
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